Thermionic cathode for electron tubes and method for producing same



M y' 1959 G. J. EKKERS ETAL 2,887,413

THERMIONIC CATHODE FOR ELECTRON TUBES AND METHOD FOR PRODUCING SAME Filed Nov. 19, 1956 2 Sheets-Sheet 1 INVENTORS W01 m4 aw,

Pm W? PM ATTORNEYS y 9, 1959 K G J. EKKERS ETAL 2,887,413

THERMIONIC C ATHODE FOR ELECTRON TUBES AND METHODFOR PRODUCING SAME Filed Nov. 19, 1956 2 Sheets-Sheet 2 wmzgwa mm M d/ZLLQMQ. BY M, 1 q- We) ATTORNEYS United States Patent 6 THERMIONIC CATHODE FOR ELECTRON TUBES AND METHOD FOR PRODUCING SAME Gysbert Jacob Ekkers, Wettingen, and Aldo Patriarca,

Laufohr, near Brugg, Switzerland, assignors to Patelhold Patentverwertungs- & Elektro-Holding A.-G., Glarus, Switzerland, a joint-stock company Application November 19, 1956, Serial No. 623,030

Claims priority, application Switzerland December 17, 1954 3 Claims. (Cl. 117212) The invention relates to a thermionic cathode for evacuated, gas or vapor-filled electrical discharge tubes.

The cathode of the present invention is distinguished from known cathodes by its high capacity which is limited only by the melting point of the cathode metal, by the complete absence of sparking even at high anode voltages, by its insensitivity to ion bombardment whereby, in gas filled tubes, the anode voltage may be applied simultaneously With the cathode heating voltage, by the absence of easily vaporizable material whereby the cathode is not a source of harmful deposits in other parts of the tube, and by its long life. Moreover, after the cathode is formed it need not be activated and artificially aged.

The cathode of the present invention is characterized in that it presents a clean metallic surface and that the grain boundaries between the crystal structure of said metal adjacent said surface contain an oxide of said metal, which metal oxide is associated with a stoichiometric excess of free atoms of said metal.

Investigation of cathodes of the invention shows that the metal oxide inclusion in the grain boundaries extends to a depth of between about 0.001 and 0.01 mm. into the body of the cathode from said surface. The stoichiometric surplus of free metal atoms associated with said oxide cannot be determined as this metal is the same as the core metal.

The emission mechanism of the cathode of the invention is similar to that of known oxide cathodes because the stoichiometric surplus of metal atoms causes the metal oxide with which it is associated to be an excesssemiconductor. The cathode is, however, not an oxide cathode as generally understood because the term oxide cathode means a metal cathode the surface of which is covered by a layer of alkaline earth metal oxide whereas the surface of the cathode of the present invention is free of such a coating.

The advantages of the cathode of the present invention compared with known cathodes are attributed principally to the fact that the vaporization rate of its constituents at operating temperature is negligible compared to that of the known oxide cathodes and that the surface is free of weakly adherent particles.

The cathode may be made of metals of the groups Vb, VIb, or VIIIb of the short-periodic system which form stable oxides which have a low vapor pressure at the operating temperature to which the cathode is subjected. It is advantageous from the standpoint of capacity to select a metal having a relatively high melting point, for example, a melting point above 1200 C. From the standpoint of capacity, molybdenum has given the best results. At an operating temperature of the cathode of 1850 C., at which a service life of at least 500 hours is to be expected, a current density of 1750 a./cm. may be attained. The Work function at this temperature is 2.48 e.v. For low temperature cathodes nickel is the most advantageous cathode metal. At 1050' C. the

2,887,413 Patented May 19, 1959 work function is 1.81 e.v. and the current density, in

spite of the lower temperature, still amounts to 30 a./sq.cm. The cathode of the invention can be operated at still lower temperatures, without harm, so that any desired service life can be achieved by selection of the operating temperature.

The cathode of the invention may be made by heating the surface of the selected metal base pointwise in the presence of oxygen to a temperature of at least 3500 C. This temperature would of course melt the metal base if applied over a sufficiently large area for a sufliciently long time, but when applied at a point which moves over the surface of the base, the surface is made active without melting the base. Each point of the surface is heated for only a few milliseconds. The base may be in any desired form such as a wire, a tape, a wire net, or a sheet. An especially advantageous method of effecting the pointwise heating of the metal base in the presence of oxygen is to coat the base with a layer of an alkaline earth metal oxide or a mixture thereof and then to apply an electric arc to the coated surface in an atmosphere of an inert gas. The are is ignited by heating the coated base. After the arc has been started the temperature is reduced until the arc concentrates upon a point on the surface of the coated metal base. As the are continues to impinge at this point the temperature at the point quickly rises to above 3500 C. and the alkaline earth metal oxide coating is removed therefrom. The heat applied to the oxide coating at the point of impingement of the are not only removes the coating at this point but also decomposes or dissociates some of the alkaline earth metal oxide thus liberating the oxygen which is necessary to form the oxide of the cathode metal. The cathode metal atoms required for association with the cathode metal oxide impregnation of the surface of the metal base are supplied by the vaporization and ionization of some of the metal of the base at the point of impingement of the arc. The metal ions are shot into the oxide lattice. As the alkaline earth metal oxide coating is removed from the surface of the metal base at the point of impingement of the arc, the work function increases and the arc moves to an adjacent point on the cathode where the alkaline earth metal oxide coating is still present and the operation is repeated at this point. This movement of the are over the surface of the cathode continues until the whole surface of the cathode has been treated i.e. until the alkaline earth metal oxide coating has been removed and the surface layer of the cathode has been impregnated with the oxide of said metal as described above. Thus the whole cathode surface is heated point-wise to a temperature which is far above the melting point of the cathode metal.

In practice, the above described procedure is carried out, using the metal cathode base, coated with alkaline earth metal oxide as the cathode for an electric are discharge, both said cathode and the anode of the are being enclosed in a chamber filled with an inert gas such as mercury vapor or one of the rare gases at a low vapor pressure such as 0.02 to 0.1 mm. Hg. To facilitate the ignition of the arc, the cathode is heated up to a temperature of say 800 C. As the alkaline earth metal oxide coating, the pure oxide of one of the alkaline earth metals may be used but a mixture such as a mixture containing at least about 40% of barium oxide with from 0 to 60% of strontium oxide and/or 0 to 15% of calcium oxide is preferred. The current density in order to effect adequate heating of the cathode surface depends upon the cross-section of the cathode metal, and may vary from 0.5 to 4 a./cm. Dependiung upon the current density employed, the cathode surface may vary in color, for instance from white to golden yellow for nickel as cathode metal. when molybdenum is the cathode metal the color generally is from white to gray. The

color of the cathode depends upon the metal of the cathode, the colorof the metal oxide coating and the thickness of the metal oxide coating.

The invention is illustrated in the accompanying drawings in which Fig. 1 is a cross section of a wire-shaped cathode, Fig. 2 is a diagrammatic representation of a highly magnified plan view of a fragment of the treated cathode surface, and

Fig. 3 is a diagrammatic illustration of apparatus in which the method of "making the cathode, described above, may be carried out.

, Referring to Figs. 1 and 2 of the drawings, 1 is the metal cathode base, 2 is the roughened or granular'cathode surface, 4 are crystals of the cathode metal, and 3 is the mpregnation of the cathode metal oxide associated with an excess of atoms of the cathode metal.

, Referring to Fig. 3, 5 is a V-shaped nickel cathode which may be, for example 7 centimeters long, 0.4 'centi in-a gastight glass tube 6 which contains also the nickel anode 7. The form, size and distance of the anode from' the cathode may vary within a wide range. For instance it may be a circular flatdisc 2 centimeters in diameter and spaced 2 centimeters from the end of the cathode. The tube 6 contains sufficient mercury to provide a vapor pressure thereof of 0.05 mm. The cathode is then heated to about 800..C. by current supplied by the transformer 8 .which is controlled by the regulator 9. 10 represents a directipurrentsource from which current, is supplied through the regulator 11 and the ammeter 12 to the anode 7. About 100 volts is applied to initiate the 'are between the anode and the cathode. The current flow is thenregulated to about 6 amperes which corresponds: to current density of somewhat more than 1 ampere per square centimeter of cathode surface. The current density. should be at least 0.5 ampere per square centimeter and generally ismaintained within the range, from 1 to 1.5 amperes per square centimeter. .Finally the heating current is so far reduced that the cathode temperature is about 550 C. The electric arc concentrates .upon' 'a small area of the cathode surface and quickly heats this area to the necessary temperature of over 3500 C. The

oxide coating quickly disappears from this area and the.

arc moves to another area where the operation is repeated and so on until, in a short time the whole surface of the cathode will have been heated pointwise to more than 3500 C. and it will be observed that the removal of the alkaline earth metal oxide coating is complete and that the cathode is completely formed and may be removed from the tube 6.

As appears from the foregoing disclosure, the cathode is operable at temperatures below 1000 C. e.g. 800 C. and is free of any layer or coating of alkaline earth metal oxide and its attendant disadvantages.

The emission of the cathode in accordance with the invention is equal to that of the known oxide cathodes and has been pointed out, has many advantages over the oxide cathode. It can be loaded to saturation without danger, since an intermediate layer which would reduce the loading capacity of the cathode does not form during operating. Activation is unnecessary. The cathode is relatively insensitive to impurities and may therefore be stored in the air in the interval between its manufacture and use. The cathode surface is free of poorly adherent particles and can therefore be used in tubes which are to be operated at high anode voltages. The vaporization of metal from the cathode is negligible. Sensitivity to ion bombardment is not greater than that of a pure metal cathode. In the case of gas-filled tubes, the anode voltage and'the heating voltage may be applied simultaneously. Preheating and activation are not necessary.- Electron release requires less than 2 electron volts. '1 Fifi,

This application is a continuation-in-part of my application Serial-No. 488,439, filed February 15, 1955, and now abandoned.

We claim:

1. A method for the production of a cathode for an electron tube which comprises coating a surface of a piece of metal of the groups Vb, VIb or VIIIb of the short-periodic system with an alkaline earth metal oxide and successively heating relatively small areas of said surface by means of an electric arc to a temperature of at least 3500 C.,whereby said coating of alkaline earth metal oxide is removed from said surface.

2. An incandescentcathode for an electron tube cond sisting of apiece of metal of the groups Vb, VIb or VIIIb of the short-periodic system, said piece of metal having a .clean 'metallic surface which is free of metal oxide coating and which is impregnated at the grain boundaries only with an oxide of said metal containing a stoichiometric excess of'atoms of said metal.

3. A cathode as defined in claim 2 in which said metal oxide extends into said piece of metal from said surface to a depth of about 0.01 mm.

References Cited in the file of this patent UNITED STATES PATENTS 1,926,846 Giard Sept. 12, 1933 2,147,447 Kolligs Feb. 14, 1939 2,536,673 Widell Jan. 2, 1951 2,556,254 Carne June 12, 1951 2,677,071 Came Apr. 2 1954 

1. A METHOD FOR THE PRODUCTION OF A CATHODE FOR AN ELECTRON TUBE WHICH COMPRISES COATING A SURFACE OF A PIECE OF METAL OF THE GROUPS VB, VIB OR VIIIB OF THE SHORT-PERIODIC SYSTEM WITH AN ALKALINE EARTH METAL OXIDE AND SUCCESSIVELY HEATING RELATIVELY SMALL AREAS OF SAID SURFACE BY MEANS OF AN ELECTRIC ARC TO A TEMPERATURE OF AT LEAST 3500*C. WHEREBY SAID COATING OF ALKALINE EARTH METAL OXIDE IS REMOVED FROM SAID SURFACE. 